High Voltage Direct Current (HVDC) power transmission has become increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems. Power systems such as electrical power distribution or transmission systems generally include a protection system for protecting, monitoring and controlling the operation and/or functionality of other components included in the power system. Such protection systems may for example be able to detect short circuits, overcurrents and overvoltages in power transmission lines, transformers and/or other parts or components of the power system. Such protection systems can include protection equipment such as circuit breakers for isolating any possible faults for example occurring in power transmission and distribution lines by opening or tripping the circuit breakers. After the fault has been cleared the power flow can be restored by closing the circuit breakers.
In a HVDC power system there is generally included an interface arrangement including or constituting a HVDC converter station, which is a type of station configured to convert high voltage DC to AC, or vice versa, which interface arrangement configured to couple an AC power system with a DC power system, or vice versa. A HVDC converter station may comprise a plurality of elements such as the converter itself (or a plurality of converters connected in series or in parallel), one or more transformers, capacitors, filters, and/or other auxiliary elements. Converters may comprise a plurality of solid-state based devices such as semiconductor devices and may be categorized inter alia depending on the type of switches (or switching devices) which are employed in the converter as line-commutated converters (LCCs), using e.g. thyristors as switches, or voltage source converters (VSCs), using transistors such as insulated gate bipolar transistors (IGBTs) as switches (or switching devices). A plurality of solid-state semiconductor devices such as thyristors or IGBTs may be connected together, for instance in series, to form a building block, or cell, of an HVDC converter. The converter cell may in alternative be referred to as a (HVDC) converter valve.
Different HVDC configurations or topologies may be used. As known in the art, a HVDC power transmission arrangement may be configured so as to have one pole, i.e. so as to exhibit a monopole configuration, two poles, i.e. so as to exhibit a bipole configuration, or possibly even more than two poles. A monopole configuration may be a symmetric monopole configuration, where the converters are connected pole to pole, and with the midpoint of the connection between the converters grounded so as to make a potential reference for the DC voltage, or an asymmetric monopole configuration, where the converters are connected pole to ground, and where a metallic return line can be used with a ground connection for making a potential reference for the DC voltage. A bipole configuration can be considered essentially as a combination of two asymmetrical monopole configurations.